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SUMMARY REPORT
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Publication Number: FHWA-HRT-05-001

SUMMARY REPORT-Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways

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As traffic volumes grow on urban freeways, highway agencies face an ongoing challenge to maintain efficient traffic operations and acceptable levels of service. Increasing the capacity of a freeway by adding a lane can be difficult and expensive if it involves widening the existing roadbed, regrading roadside areas, and/or acquiring additional right-of-way. A number of highway agencies, however, have implemented projects in which a travel lane is added on an urban freeway by restriping the traveled way with narrower lanes, converting all or part of the shoulder to a travel lane, or a combination of both. The traffic operational benefits of such conversions are immediate and obvious, but the safety effects are uncertain. This study addresses these safety effects.

Literature Review

McCasland(1) evaluated two freeway segments in Houston, TX on which narrower lanes and a narrower outside shoulder were used to create an additional travel lane. Reductions in the accident rate per million vehicle-kilometers (veh-km) were found using a Poisson comparison of means test. Urbanik and Bonilla(2) evaluated similar projects on urban freeway segments in California using a two-sample t-test. Statistically significant changes in the accident rate were found for three of the 10 projects evaluated. Two projects experienced statistically significant reductions in the accident rate, but one project experienced a statistically significant increase. In particular, the entire accident rate increase for this project occurred near the downstream end of the segment. There are concerns that both evaluations addressed accident rate rather than accident frequency and did not compensate for regression to the mean, both of which could have distorted the safety benefits.

The current study attempts to account for these possible biases.

Methodology

Databases Used

Because of the need for adequate periods both before and after treatment, the study analyzed data on urban freeway sites in California-78.7 kilometers (km) (48.9 miles(mi)) of treatment sites, 31.1 km (19.3 mi) of untreated sites downstream from treatment sites, and 398.5 km (247.6 mi) of untreated reference sites similar to the treatment sites. Supplemental data was also collected on sites immediately upstream of the treatment sites. Sites in all three groups had median barriers present. All crash, traffic volume, and roadway inventory data were extracted from 1991-2000 California files in the Federal Highway Administration's (FHWA) Highway Safety Information System (HSIS).

Research Design

All treatments involved converting either four lanes in one direction to five lanes, or five lanes in one direction to six lanes. New travel lanes were developed from existing pavement width by converting paved shoulders to travel-lane width, narrowing existing lanes by restriping, or a combination of the two. The treatments had various combinations of before-and-after geometrics (i.e., different amounts of shoulder conversion and lane narrowing), which were grouped into "bundles" for analysis. The primary bundles in both classifications involved narrowing only the inside shoulder, and either leaving the lane width at 3.7 meters (m)(12 feet (ft)) or narrowing the lanes to at least 3.4 m (11.2 ft). In all but one bundle, the added lane was a high occupancy vehicle (HOV) lane during at least some part of the day.

The goal of the study was to examine the effects of the treatment on three measures of effectiveness:

  • Total accidents (fatal, injury, and property-damage-only (PDO) accidents, including both towaway and non-towaway accidents).
  • Fatal, injury, and PDO towaway accidents (excluding PDO non-towaway accidents).
  • Fatal and injury accidents (excluding all PDO accidents).

In addition to examining changes in these three measures for all four- to five-lane conversions combined and all five- to six-lane conversions combined, the authors also attempted to examine the effects of different "bundle" types, different crash types (e.g., sideswipe crashes), and the number of interchange ramps in a section. They also studied possible upstream effects of changes in flow and downstream effects of "accident migration" when the new lane was dropped.

All treatments were implemented in 1993, and crash, traffic, and inventory data were available in HSIS from 1991-2000, so the data allowed for a before-and-after study. To control for possible regression to the mean and other biases, the empirical Bayes (EB) methodology described by Hauer(3,4) was used. Here, a prediction of what would have happened at the treatment sites in the after period without treatment is based on a weighted combination of two factors: (1) the frequency of crashes on the treated sites in the before period, and (2) crash-frequency predictions from regression models developed with data from the untreated reference sites. The prediction of what would have happened without treatment is then compared to what actually happened with treatment to estimate the safety effect of the treatment. The methodology corrects for the regression bias, changes in traffic volume at the treatment sites, and other possible confounding factors. Details of the methodology are in the paper referenced at the end of this summary.

Data Collection

As noted above, crash, inventory, and traffic data for treatment, downstream, and reference sites were extracted from the 1991-2000 California HSIS files. Table 1 presents basic descriptive statistics for the three types of sites, and table 2 presents the crash data for the before-and-after periods at the treatment sites. For all three types, the mileage was divided into "homogeneous sites" for analysis purposes, with each site being homogeneous for conversion type (number of lanes for the reference sites) and traffic volume. All of the treatment and downstream sites were located in two southern California counties, and the reference sites were located in these two counties plus four surrounding counties.

Table 1. Descriptive statistics of evaluation sites.

TYPE OF SITE
NUMBER
OF
LANES
NUMBER
OF
SITES
TOTAL
LENGTH
OF SITES
(MI)
AADTa (VEH/DAY) (1994) NUMBER OF RAMPS
MINIMUM MEAN MAXIMUM ON-RAMPS OFF-RAMPS TOTAL
Treatment
  4 to 5b
79
36.4
79,000
104,081
128,000
60
51
111
  5 to 6b
45
12.5
77,000
107,497
126,500
14
15
29
  Total
124
48.9
77,000
104,951
128,000
74
66
140
Downstream
  4 to 5c
45
11.4
62,5000
103,267
128,000
28
23
51
  5 to 6c
33
7.9
77,000
114,121
126,500
14
19
33
  Total
78
19.3
62,500
107,859
128,000
42
42
87
Reference
  3d
92
45.7
5,600
63,958
142,500
205
222
427
  4d
270
138.6
14,250
79,965
164,000
559
534
1,093
  5d
128
63.4
48,500
109,245
164,000
154
149
303
  Total
490
247.7
5,600
81,227
164,000
918
905
1,823
a Annual average daily traffic volume (veh/day) for one direction of travel. 1 mile = 1.6 kilometers
b Number of lanes before and after the project (i.e., conversion type).
c Number of lanes before and after the project on the adjacent treated site.
d Number of lanes on the reference site.

Analysis

As noted above, one component of the prediction of after-period accident frequencies at the treatment sites without treatment is a regression model (i.e., a safety performance function (SPF)) developed using data from the untreated reference sites. In this study, SPFs using a negative-binomial distribution were developed with the following primary independent variables:

  • Annual average daily traffic (AADT).
  • Segment length.
  • AADT and segment length combined into a single exposure variable as EXPO equals (AADT times segment length) divided by 106 in million vehicle miles (MVM).

Examination of several model forms indicated that the most appropriate and useful models had the following form:

Expected number of accidents per year = exp(beta symbol1) x AADTbeta symbol2 (segment length)

The regression coefficients beta symbol1(intercept) and beta symbol2(exponent of AADT), the overdispersion parameter of the negative binomial distribution, and two goodness-of-fit measures (i.e., the ordinary multiple correlation coefficient, R2, and the Freeman-Tukey coefficient, RFT2) were estimated by the method of maximum likelihood using a commercially available SAS® statistical analysis software named PROC GENMOD.(5)

While the EB approach compensates for regression to the mean and adjusts for the effect on safety of changes in AADT over time, the effect on safety of changes in other factors over time (e.g., accident reporting practices, demography, weather) also needs to be addressed. This was accomplished by developing a series of yearly calibration factors to ensure that the SPF-predicted and observed accidents at each treated site during the before period are the same(4) and using these calibration factors to adjust the predicted accidents for each specific year. For the examination of off- and on-ramp effects, modified SPFs including an independent variable for the number of ramps were developed.

Results

Estimated Safety Effects of Four- to Five-Lane and Five- to Six-Lane Conversions

The results of the primary analyses for different crash injury levels within the two categories of treatment sites are shown in table 3. Note that the designation of the statistical significance of the change in crash frequency is based on the ratio of the mean treatment effect to its standard error. Hauer(3) recommends that a ratio of 2.0 or greater be used in judging the results of the EB analysis. Although not a formal test of significance, this could be equated to an approximate 98 percent (one-sided) test.

The EB analysis results in table 3 indicate that the four- to five-lane conversions, on the average, resulted in a statistically significant increase in accident frequency of 10 to 11 percent. The five- to six-lane conversion projects resulted in an increase in accident frequency of 3 to 7 percent, not statistically significant. The sample size for five- to six-lane conversions was about half that for four- to five-lane conversions, so the five- to six-lane analysis would be less likely to result in statistically significant results as reflected by the larger standard errors.

Table 2. Accident frequencies at treatment sites.

CONVERSION
TYPE
BEFORE PERIOD (1991-1992)
NUMBER OF ACCIDENTS AVERAGE
AADTa
(VEH/DAY)
EXPOSURE
(106 VEH-MI)
FATAL INJURY PDO TOWAWAY PDO NON-TOWAWAY TOTAL
4 to 5 lanes 8 629 201 947 1,785 105,461 2,804.5
5 to 6 lanes 2 243 71 340 656 110,605 1,020.0
Total 10 872 272 1,287 2,441 106,772 3,824.5
a AADT is for the treated direction of travel only.
CONVERSION
TYPE
AFTER PERIOD (1994-2000)
NUMBER OF ACCIDENTS AVERAGE
AADTa
(VEH/DAY)
EXPOSURE
(106 VEH-MI)
FATAL INJURY PDO
TOWAWAY
PDO
NON-TOWAWAY
TOTAL
4 to 5 lanes 26 2,310 2,204 3,103 7,643 107,267 9,983.7
5 to 6 lanes 13 809 731 1,048 2,601 111,874 3,613.3
Total 39 3,119 2,935 4,151
10,244
108,441 13,597.0
a AADT is for the treated direction of travel only.

Supplemental Results

The examination of crash types (both changes in single- and multivehicle proportions and in individual crash types) indicated no statistically significant change on the treatment sites. In general, the results show that the frequency of rear-end collisions increased after project implementation. The frequency of sideswipe accidents increased for the four- to five-lane conversions, but decreased for the five- to six-lane conversions (which may help explain the difference in effects in the primary analysis above).

The analyses of the individual "bundle" types did not show differences between bundles that could be used in deciding how best to apply the treatment. The analysis of ramp locations did not show statistically significant results, but it was interesting to note that crash frequency increased both near and away from ramps on the four- to five-lane conversions and near ramps in the five- to six-lane conversions. Crash frequency decreased in the five-lane conversion sites away from ramps.

The examination of possible "accident migration" to adjacent downstream sites indicated a nonsignificant increase for the four- to five-lane conversions of 5 to 12 percent, and a statistically significant 17 to 21 percent increase downstream from the five- to six-lane conversions. An effect that potentially offsets the accident migration on the five- to six-lane conversions was a nonsignificant decrease in crash frequencies for freeway segments upstream of the conversion site.

Table 3. Empirical Bayes analysis results for primary evaluation of specific conversion types.

CONVERSION
TYPE
MEASURE OF EFFECTIVENESS/
DEPENDENT VARIABLE
NUMBER OF
SITES
PERCENT CHANGE IN ACCIDENT FREQUENCY RATIOb SIGNIFICANT?c
MEANa STANDARD ERROR
4 to 5 lanes
  Total accidents 79 10.96 2.88 3.8 Yes
  Fatal, injury, and PDO
towaway accidents
78 9.67 3.89 2.5 Yes
  Fatal and injury
accidents
78 10.59 4.56 2.3 Yes
5 to 6 lanes
  Total accidents
43
3.02
4.56
0.7
No
  Fatal, injury, and PDO
towaway accidents
45
3.71
6.08
0.6
No
  Fatal and injury
accidents
45
7.08
7.22
1.0
No
a A positive mean percent change indicates an increase in accident frequency, and a negative mean indicates a decrease.
b Ratio of mean percent change in accident frequency to standard error of percent change in accident frequency.
c Significant if ratio greater than or equal to2, and not significant if ratio <2.

Discussion

The analysis results indicate that narrow-lane or shoulder-use-lane projects on urban freeways increase accident frequencies for four- to five-lane conversion projects. Such conversions may increase accident frequencies for five- to six-lane conversion projects as well, but the results for those projects were not statistically significant. Because of the different findings for these two types of conversions, the results obtained are difficult to generalize to urban freeways as a whole.

One possible explanation for the increase in accident frequency on conversion projects is that the added lanes in most of the projects were HOV lanes. Speed differentials between the main lanes and HOV lanes on freeways have the potential to increase sideswipe and lane-changing accidents, although this effect has not been satisfactorily quantified in the literature. The crash type results in this study indicated a nonsignificant increase in sideswipe collisions on the four- to five-lane conversions, but a decrease on the five- to six-lane conversions. If this is indeed true, it may help explain why the results differ between the two classes.

The results also suggest that, at least for the five- to six-lane conversions, the effect of the project may have been to dissipate congestion upstream of the treatment site by removing the treatment site as a bottleneck. It is possible that the effects of the four- to five-lane conversions have been partially because of the displacement of a bottleneck as well. The bottleneck may have been transferred to a location downstream of the treatment site, with a corresponding increase in accident frequency at that location and possibly within the treatment site itself.

In summary, the findings are more complex than expected. Differences may exist in the crash-related effects of lane conversion treatments at four-lane versus five-lane sites. The differences between road classes observed may be explained by differences in traffic operations (e.g., speeds, lane-changing behavior) that could not be analyzed in this study. In addition, the observed increases in accident frequency cannot necessarily be attributed to the use of narrower lanes or the conversion of a shoulder to a travel lane. The use of the added lanes as HOV lanes, which may introduce a difference in speed between adjacent lanes, may be another explanation for the increase in accidents. The analysis results also suggest that the conversion projects may decrease accident frequencies upstream of the project and increase accident frequencies within and downstream of the project because the projects may result in the relocation of a traffic operational bottleneck. These various effects on safety are confounded in the data and could not be separated in this study.

References

  1. McCasland, W. R., "Use of Freeway Shoulders to Increase Capacity," Transportation Research Record 666, Transportation Research Board, Washington, DC, 1978, pp. 46-51.
  2. Urbanik, T., Bonilla, C. R., "California Experience with Inside Shoulder Removals," Transportation Research Record 1122, Transportation Research Board, Washington, DC, 1987, pp. 37-36.
  3. Hauer, E., Observational Before-After Studies in Road Safety, Pergamon/Elsevier Science, Inc., Tarrytown, NY, 1997.
  4. Hauer, E., Council, F.M., and Mohammedshah, Y., "Safety Models for Urban Four-Lane Undivided Road Segments," Presentation, 8th Annual Meeting of the Transportation Research Board, Washington DC, January 2005. Accepted for publication in Transportation Research Record (Draft in CD of Annual Meeting of the Transportation Research Board, 2004).
  5. SAS/STAT® User's Guide, Version 8, SAS Institute Inc., Cary, NC, 1999, pp. 1,363-1,464.

For More Information

This research was conducted by Karin M. Bauer, Douglas W. Harwood, and Karen R. Richard of the Midwest Research Institute and Warren E. Hughes of BMI-SG. The final report, "Safety Effects of Using Narrow Lanes and Shoulder-Use Lanes to Increase the Capacity of Urban Freeways," appears in the Transportation Research Board's Transportation Research Record: Journal of the Transportation Research Board No. 1897, 2004.

For more information about HSIS, contact Carol Tan, HSIS Program Manager, HRDS, 202-493-3315, carol.tan@fhwa.dot.gov.

The Highway Safety Information System (HSIS) is a multi-State safety database that contains crash, roadway inventory, and traffic volume data for a select group of States. The participating States—California, Illinois, Maine, Michigan, Minnesota, North Carolina, Ohio, Utah, and Washington—were selected based on the quality of their data, the range of data available, and their ability to merge the data from the various files. The HSIS is used by FHWA staff, contractors, university researchers, and others to study current highway safety issues, direct research efforts, and evaluate the effectiveness of accident countermeasures.

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